The disclosed subject matter relates to techniques for hyperspectral imaging.
Different materials absorb and emit electromagnetic radiation in corresponding different manners, and this spectrum can serve as a fingerprint for a particular material, helping to identify it from others. The ability to visualize multiple frequencies of the spectrum can permit the material composition of the imaged region to be deduced. Unfortunately, certain conventional imaging technologies can provide limited spectral information, focusing on a few frequency ranges.
A hyperspectral imager can allow the visualization of many frequencies and, with adequate post-processing, can provide detailed information about the material composition of the environment or sample. As a result, hyperspectral imagers can be used to identify weapons, biological agents and chemicals, among other things.
In general, certain approaches to hyperspectral imaging can include using a one-dimensional spectrometer resolved into a 2D plane. That is, for example, one dimension can be a spatial dimension and the other dimension can be a spectral dimension. However, in certain instances it can be useful to simultaneously resolve spatial information and spectral information in at least two dimensions, for example where comparison of spectral signatures at discrete spatial locations is desired. In certain other techniques, a tunable optical filter can be used in connection with an imaging array. For example, the tunable optical filter can be tuned over various bands in a spectrum, and at each band an image can be acquired at each pixel in the array, thereby providing a spectral range.
Certain approaches to hyperspectral imaging can be complex, bulky, expensive, and can include various limitations. For example, certain hyperspectral imaging technology can rely on large, expensive optical assemblies such as diffraction gratings or prisms; similarly, optical modulation techniques such as acousto-optic tunable filters or liquid crystal tunable filters can be used. Certain other approaches can involve the use of imaging integrated circuits with patterned color filters on the surface, integrated pixel-arrays, photonic crystal arrays, tunable band gap semiconductor detectors, and MEMS-based tunable cavity arrays.
Certain approaches, such as the use of patterned color filters arrays, integrated pixels arrays, and photonic crystal arrays, can have fixed spectrum depth due to the fact that the filters are not tunable. Other approaches, such as e.g., the use of photonic crystals, MEMS, or tunable band gap semiconductor technologies can be ill-suited for general-purpose sensing or imaging applications, have low detection efficiency, and can be difficult to integrate with existing imaging optics. Still other approaches, e.g., MEMS-based approaches, can suffer from performance deterioration over time owing to mechanical wear and electrostatic attraction between individual components.
Certain approaches to hyperspectral imaging, can include the use of Fabry-Perot interferometers to obtain spectral ranges for pixels of an imaging array. In this manner, spatial information (e.g., via a plurality of pixels) can be obtained, as well as spectral information. For example, stacks of Si/SiNx can be used as broadband mirrors, disposed around an air cavity, and can create a resonant cavity. Electrodes can be placed on the outer side of the mirrors, and a voltage can be applied to create a capacitance between the mirrors, and thereby modulate the distance between the mirrors via capacitive forces. Example techniques are disclosed in U.S. Pat. Nos. 7,355,714, 7,385,704, and 6,295,130, each of which is hereby incorporated by reference in its entirety.
However, controlling the distance between the mirrors using voltage applied to the electrodes can be difficult. For example, due to the quadratic dependence of displacement on applied voltage of the spring-capacitor system, precise control of a system's displacement can be difficult due to an inherent nonlinearity. Special control circuitry can be required to attain finer control.